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Progressive Image Rendering: Good or Evil?

Tammy Everts (SpeedCurve), Kent Alstad (Radware Canada)
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Slides:   external link

Everyone has strong opinions about progressive images, but there’s little to no hard evidence that proves whether progressive image rendering helps or hurts the user experience. It’s time to end the data-free analysis so we can all get on with our lives.

Some factions believe that by loading images progressively, we improve perceived performance by showing the user something while they look at the screen. Others believe that watching an image load progressively increases user frustration. We took these assumptions to the lab to see if we could definitively answer the question: do progressive images deliver a better or worse user experience? 

Using a proven neuroscientific approach that we pioneered in a previous study, we served test participants with multiple versions of a set of pages for an ecommerce workflow. Each set of pages rendered images differently:

  • Baseline images, normal speed
  • Baseline images, throttled speed
  • Progressive images, normal speed
  • Progressive images, throttled speed

Using automated facial coding technology that measures moment-by-moment emotional responses in facial micro-expressions, we extracted data about users’ perception of these experiences. Using Implicit Priming Testing, we also extracted measures of frustration and emotional engagement from the experiences. 

Using this data, we were able to answer questions such as:

  • Is there a correlation between image type/quality and the perceived user experience?
  • Does image quality matter more on some pages than on others (e.g. home page vs product page)?
  • What is the bottom-line threshold for user tolerance of slow images?
  • At what point does superior image quality cease to offer additional benefit to the user experience?
  • Using anecdotal feedback from participants’ exit interviews, are people conscious of differences in image speed/rendering?

The results of this study have huge ramifications:

  • Ending the highly subjective data-free debate over whether progressive images help or hurt the user experience.
  • Potentially saving tons of developer time in manually optimizing images.
  • Feature development in automated front-end optimization tools — like ours! (Disclosure: Progressive image rendering is a feature we offer in our own solutions, so this research was a nail biter for us. In the past, we’ve drunk the progressive JPEG Kool-aid, but we’re ready to abandon or modify this feature if it means delivering the best possible user experience to our customers’ users.)
  • Ultimately serving the best — and fastest — experience to users.
Photo of Tammy Everts

Tammy Everts


As a senior researcher, writer, and performance evangelist at Radware, I’ve spent the past few years researching the technical, business, and human sides of web/application performance, and sharing the findings via countless blog posts, presentations, case studies, articles, reports, and infographics. Before joining Radware, I was research lead and senior writer at Strangeloop Networks (which was acquired by Radware in early 2013).

I blog at both the Radware blog and Web Performance Today.

Photo of Kent Alstad

Kent Alstad

Radware Canada

Kent is an entrepreneur, software architect, and technology innovator. Before taking the role of VP Acceleration at Radware, Kent was CTO at Strangeloop Networks, where he was instrumental in authoring all of Strangeloop’s issued and pending patents. Prior to helping create Strangeloop, he served as CTO at IronPoint Technology. Kent also founded Eclipse Software, a Microsoft Certified Solution Provider, which he sold to Discovery Software in 2001.

In more than 25 years of professional development, Kent has served as architect and lead developer for successful production solutions with The Active Network, ADP, Lucent, Microsoft, and NCS. ”Port View”, an application Kent architected for the Port of Vancouver, was honoured as Best Administrative System at the 1996 Windows World Open Competition.

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Alex B
09/16/2014 3:25pm EDT

I’m doing a talk about technical implementation of progressive image loading in Melbourne next month. Would really love to see the slides!

Picture of Tammy Everts
Tammy Everts
09/16/2014 4:59am EDT

We’ll be sharing them at Velocity this afternoon (Tuesday) at 3:30, Alexander. After our session, we’ll post our slides in this space.

Alex B
09/16/2014 4:35am EDT

So what are your findings?